For the purpose of receiving and regenerating optical signals after passage through a transmission path, digital optical receivers are known which comprise a receiver circuit, in which the optical signal is converted into an analog electrical signal, and a digital signal processing part, in which the analog signal is regenerated into a digital data signal with standardized amplitude and clock information. In the receiver circuit, the received optical signal is converted into a photocurrent by means of a photodiode and amplified in a preamplifier.
Modern optical receiver circuits have to satisfy high requirements with regard to their dynamic range. As a rule, in order to convert the photocurrent into voltage values, current-voltage converters, what are known as transimpedance amplifiers (TIA), are used. Here, the entire photocurrent flows via a feedback resistor and is present as a voltage at the output of the transimpedance amplifier.
In order to cover a desired dynamic range of a few 10 dB, it is known to connect an MOS transistor in parallel with the feedback resistor, said transistor being operated in triode mode and its gate voltage being regulated in accordance with the level of the input signal from the photodiode and the dynamic requirements. In this way, a linear resistance parallel to the feedback resistor is provided. Since the resultant total resistance value decreases because of the parallel connection of two resistors, the gain of the transimpedance amplifier also decreases. A lower gain is associated with a reduced stability and phase margin of the transimpedance amplifier and can lead to undesired oscillations.
It is also known, in order to avoid such oscillations and to maintain the stability, either to reduce the open-loop gain or else to connect a variable compensation capacitor in parallel with the nonreactive feedback resistor and the MOS transistor. The first alternative is generally not practical. With regard to the second alternative, in the prior art, solutions via a plurality of comparators and a plurality of reference voltages for these comparators are connected up step-by-step to compensation capacitors. This disadvantageously entails high expenditure on circuitry. In addition, the control behavior is discontinuous.
There is a need for an amplifier circuit for converting the current signal from an optical receiving element into a voltage signal, using a transimpedance amplifier, which provides a high dynamic range with simultaneous stability and a continuous control behavior.